Rooftop cargo racks, otherwise simply known as roof-racks, provide a structure onto which cargo can be securely fastened above a vehicle. An advantage of a roof-rack is that it provides an option for transporting cargo that does not use space inside a vehicle. This is particularly useful for large items, such as for example, but not limited to, recreational equipment (e.g. bicycles, skies, etc.), cargo-boxes and building materials.
Despite the usefulness of roof-racks, once cargo is secured to a roof-rack on a vehicle there is the added potential for a collision between the cargo and some overhead obstruction—such as, for example only, a garage door, public parking structure, over-hanging sign and tree branch. Collisions between rooftop mounted cargo and an overhead obstruction may in turn lead to damage of the cargo, the vehicle and/or to the overhead obstruction. Unfortunately, such collisions are not simple to avoid since it is often difficult for a driver to visually assess the amount of clearance there may be, if any, between the top of rooftop mounted cargo and an overhead obstruction. Moreover, the risk of a collision may increase if a driver forgets, as a result of fatigue or routine, that cargo has been mounted to the top of the vehicle.
Most presently available collision avoidance systems are designed to detect people and/or objects in the direct forward or reverse paths of a vehicle. Only a few collision avoidance systems have been designed for detecting the presence of overhead obstructions, and in turn warning a driver of a potential collision between rooftop mounted cargo and a detected overhead obstruction. The previous systems available each have a number of drawbacks that have prevented their adoption and/or rendered the systems inadequate for their intended purpose. For example, many of the presently available collision avoidance systems employ either ultrasonic acoustic systems or Doppler-based radar systems, which are both poorly suited to the task of detecting the potential for a collision between rooftop mounted cargo and an overhead obstruction.
Ultrasonic acoustic systems often suffer from poor performance resulting from low signal directivity and a severely limited ability to filter out noise and spurious signals from vehicle vibrations and environmental factors (e.g. snow, rain, wind). Moreover, the effective volume of space through which ultrasonic acoustic signals propagate to provide usable information is quasi-spherical since ultrasonic acoustic waves have an inherent radial propagation pattern that is not easily altered. The lack of a directive signal means that ultrasonic acoustic devices cannot be focused on a specifically shaped volume of space (i.e. a 3-D zone) to the exclusion of other spaces outside the desired space. In turn, the effective volume of space from which information can be gleaned is often either too large or too small to effectively provide an accurate assessment of dangers to a vehicle, cargo and/or surrounding objects. U.S. Patent Application No. 2004/0183661 to Bowman discloses an example of a sensing system exclusively employing ultrasonic acoustic waves. Depending on the power output of the ultrasonic acoustic transmitter the system provided by Bowman can be expected to produce either too many false-positives (i.e. an indication of a possible collision that is not a real risk) or too many errors (i.e. not providing a suitable indication of an actual collision risk).
Doppler-based radar systems require relative motion in the radial direction between a radar transmitter and a target. Stationary targets or targets having a slow relative speed may not be detected at all or at least not until the vehicle is moving towards the target at a sufficiently fast speed, at which time it may be too late to avoid a collision. As a result, Doppler-based radar is not a good option for collision avoidance systems that are to be employed for determining the risk of a collision between rooftop mounted cargo and an overhead obstruction because of the poor performance of Doppler-based radar at low speeds at which most collisions of this type occur, and thus, where the collision avoidance system would be useful.